NIRISS has an all-reflective optical design that consists of:
- a pick-off mirror (POM)
- a three-mirror collimator assembly
- a dual pupil/filter wheel assembly
- a three-mirror camera assembly
- a single Hawaii 2RG detector
The optical path is illustrated schematically in Figure 1. The optical assembly is attached to an optical bench, which is shared with the Fine Guidance Sensor. The optical bench is made of aluminum. Three kinematic mounts, which are made of titanium, attach the optical assembly to the structure of the Integrated Science Instrument Module (ISIM).
NIRISS optical assembly
Figure 1. Optical path of NIRISS
A schematic plot showing the optical layout of NIRISS. Source: Honeywell.
Figure 2. Solid body rendering of NIRISS
Solid body rendering of NIRISS. Key components of the optical path are labelled. The kinematic mounts fasten NIRISS to the ISIM structure. Source: Honeywell.
Figure 3. NIRISS Flight Hardware
Photograph of the NIRISS flight hardware taken at the Goddard Space Flight Center in December 2014 between the second and third cryovacuum test campaigns. Compare with Figure 2, where key components are identified. The path between the POM (which is not visible in this image) and the collimator three-mirror assembly (TMA) is shielded by internal baffling. Source: Honeywell
The POM is a flat mirror composed of an aluminum substrate coated with nickel plating. Light from the fine steering mirror of the JWST Optical Telescope Element (OTE) is focused onto the NIRISS POM and directed into the instrument.
The POM is mounted on a movable stage that serves as the "focus adjust mechanism" (FAM; also called the Coarse Focus Mechanism [CFM]) for NIRISS.
The POM has four coronagraphic occulters engraved in its surface. These deep, cone shaped holes in the nickel overcoat are remnants of the original Tunable Filter Imager (TFI) configuration of the instrument. Although NIRISS does not have a coronagraphic mode, these occulters will nevertheless leave their imprint on all images of externally-illuminated sources. When projected onto the detector, the occulters appear as circular spots with diameters of 0.58", 0.75", 1.5", and 2.0" (approximately 9, 11. 23. and 31 pixels, respectively), with positions that depend slightly on the focus.
The NIRISS detector is a single Hawaii 2RG sensor chip array with 2048 × 2048 pixels. It provides a field of view of 2.2" × 2.2" with a plate scale of approximately 0.065 arcsec/pixel.